In the case of your equation, I'd say that there are three issues that should be addressed so that its form doesn't draw attention to itself for the wrong reasons, i.e., clumsy typography, rather than for the mathematics:

the lack of space between the right-hand end of the square root sign and the closing right parenthesis (the issue you identify in your question);

the excessive space between the opening left parenthesis and the square root sign's opener; and

a sense that the parentheses are too large and thus visually quite dominant, relative to what's inside the parentheses.

Incidentally, it's not necessary to use \dfrac for your equation; \frac works just fine. The following MWE addresses these issues:

Separately, the mathptmx package is known to generate math expressions that can look a bit cramped. If you're interested in perfecting the appearance of your formulas some more, you may want to look into using the MathTime Professional II font package. It's not a free package, unfortunately; however, its "lite" subset is free. :-) The following MWE shows what I would consider to be an even better looking version of your equation: